1) Field of the Invention
The present invention relates to the forming and bonding of structural members and, more particularly, relates to the use of fine grain titanium for superplastic forming and diffusion bonding.
2) Description of Related Art
Superplastic forming (SPF) generally refers to a process in which a material is superplastically deformed beyond its normal limits of plastic deformation. Superplastic forming can be performed with certain materials that exhibit superplastic properties within limited ranges of temperature and strain rate. For example, workpieces formed of titanium alloys are typically superplastically formed in a temperature range between about 1450° F. and 1850° F. at a strain rate up to about 3×10−4 per second.
Diffusion bonding (DB) generally refers to a process of joining members using heat and pressure to form a solid-state coalescence between the materials of the joined members. Joining by diffusion bonding occurs at a temperature below the melting point of the materials that are being joined, and the coalescence therebetween is produced with loads below those that would cause macroscopic deformation of the article.
According to one conventional process, superplastic forming is performed by providing one or more superplastically formable metal sheets in a die cavity defined between cooperable dies, heating the sheets to an elevated temperature at which the sheets exhibit superplasticity, and then using a gas to apply differential pressures to the opposite sides of the sheets in order to form the sheets. The pressure is selected to strain the material at a strain rate that is within its superplasticity range at the elevated temperature, stretch the sheet, and cause it to assume the shape of the die surface. In this way, the sheet can be formed to a complex shape defined by the dies. Further, in some cases, superplastic forming and diffusion bonding are performed in a combined forming/bonding operation. For example, in one typical combined SPF/DB process, three metal sheets are stacked to form a pack. A stop-off material is selectively provided between the sheets to prevent portions of the adjacent surfaces of the sheets from being bonded. The pack is heated and compressed in a die cavity with sufficient gas pressure so that the adjacent portions of the sheets that are not treated with the stop-off material are joined by diffusion bonding. Thereafter, a pressurized gas is injected between the sheets to inflate the pack, and thereby superplastically form the pack to a configuration defined by the surface of the die cavity. This process is described further in U.S. Pat. No. 3,927,817 to Hamilton, et al. Such a combined SPF/DB process can be used, e.g., to produce complex honeycomb sandwich structures that are formed and diffusion bonded to define hollow internal cells. Generally, the simplicity of the superplastic forming and/or diffusion bonding processes can result in lighter and less expensive structures with fewer fasteners and higher potential geometric complexity. Applications of SPF and/or DB include the manufacturing of parts for aircraft, other aerospace structures, non-aerospace vehicles and structures, and the like.
Titanium alloys are often used for superplastic forming with or without diffusion bonding and, in particular, Ti-6Al-4V (or “Ti 6-4”), which includes approximately 6% by weight aluminum, 4% by weight vanadium, and the remainder titanium. Ti-6Al-4V is conventionally superplastically formed and diffusion bonded at a temperature of about 1650° F. This relatively high forming and bonding temperature thermally stresses the dies or other tooling used during the operation. In addition, the operation of the dies at such temperatures can result in pitting or other degradation of the dies. If the surfaces of the dies that contact the sheet are pitted or otherwise damaged, the contour imparted to the sheets during the SPF/DB operation can be correspondingly nonuniform. Thus, in some cases, the dies must be regularly cleaned and dressed to maintain a particular surface finish, and such dies typically must be replaced periodically. In addition, higher forming/bonding temperatures generally put more demand on the heater that is used for heating the part to the forming/bonding temperature, and the higher temperatures also require more energy.
Conventional Ti-6Al-4V is generally not superplastically formed or diffusion bonded at temperatures below about 1650° F. At lower temperatures, a greater forming stress is required for forming the material. For example, at 1650° F., a conventional sheet of Ti-6Al-4V can be superplastically formed to twice its original length at a strain rate of 3×10−4 per second and at a true stress of less than about 5000 psi. At a temperature of about 1425° F., a similar piece of material would typically be subjected to about a 100% higher stress to achieve the same strain rate.
It has further been observed that one typical operation for superplastic forming and/or diffusion bonding of Ti-6Al-4V results in the formation of a layer of brittle alpha case oxide material on the surfaces of the titanium sheet, typically about 0.002 inch thick on each surface of the sheet. The alpha case oxide layer is normally removed from the sheet by chemical milling, i.e., exposing the sheet to an acidic liquid that dissolves the 0.003-inch thick layer of the material in about 4 minutes. As a result, a sheet having a thickness greater than the desired thickness of the finished product must typically be used to allow for the oxidation and subsequent removal of material. Also, the chemical milling operation typically does not affect the sheet uniformly, and therefore some portions of the sheet may be milled beyond the desired amount before other portions are sufficiently milled.
While the conventional methods for SPF/DB processing have proven effective for manufacturing a variety of structural parts, including parts formed of titanium, there exists a continued need for improved SPF/DB methods and parts. The methods should reduce the wear on the dies or other tooling and the required maintenance therefor. Also, the methods should reduce the required energy for SPF/DB operations and the oxidation of material during processing.